A vented rainscreen is an increasingly common exterior detail on exterior walls. Instead of attaching siding directly to the sheathing, builders add an air space behind the siding, which helps carry away moisture and prolongs the life of the siding.
Builders typically use vertical furring strips to create the gap, but there also are commercial products like Home Slicker, a plastic mesh, that are designed to do the same thing.
A rainscreen certainly is an extra step in construction, but many builders think it’s well worth the effort. So, Eric Mikkelsen asks, are vented rainscreens equally valuable on a roof?
“I’ve seen lots of info about rainscreen walls,” he writes in a Q&A post. “What about roofs? I’m planning on using a standing seam metal roof, 10/12 pitch. Should I be considering rainscreen details for this?”
Although Mikkelsen’s original post dates from 2012, the question is still sparking interest among GBA readers.
Is the attic ventilated?
James Morgan’s first question is whether the attic is ventilated, which would leave the underside of the roof sheathing open for inspection. If the attic is ventilated, Morgan says a rainscreen detail on the roof is unnecessary.
The roof, Mikkelsen says, is insulated with spray polyurethane foam applied on the underside of the sheathing, meaning the roof is not ventilated.
GBA editor Martin Holladay notes that a common approach for builders worried about ice dams — admittedly a concern that rarely plagues homes with metal roofing — is to apply 2x4s on the flat on top of the roof sheathing, with one 2×4 over each rafter or truss. Then a second layer of roof sheathing goes on, followed by roofing underlayment and the roofing. The 2x4s will create a 1 1/2-inch air gap between the two layers of sheathing. The vented channel reduces the chance of ice dams and helps keep the upper layer of sheathing dry.
An optional improvement, Holladay adds, would be to add one or two layers of rigid foam insulation between the bottom layer of roof sheathing and the 2x4s.
The foam is a continuous layer installed on the sheathing before the 2x4s are added. The insulation isn’t cut to fit between the 2x4s.
Why add a rainscreen on a “hot” roof?
To Ron Keagle, the need for a rainscreen on a roof generally doesn’t seem clear. In this case, with spray foam on the underside of the roof deck with no ventilation intended, why not just lay the metal roofing directly on top of the sheathing?
“What would be the point of creating an air space between the top of the decking and the bottom of the metal roof?” he asks.
The point, replies Morgan, is that sooner or later the roof will leak, as all roofs eventually leak. “When the underside of the roof sheathing cannot be inspected,” he adds, “a free drainage plane immediately below the roof finish ensures the sheathing has good long-term protection from concealed moisture and rot.”
Details for a metal roof
Armando Cobo offers Mikkelsen one possible way of assembling the roof (see the drawing below). It incorporates 1×4 notched battens directly beneath the metal roofing, plus 1 inch of rigid insulation and spray insulation below the roof deck. Here, the battens run horizontally.
Some builders prefer to use treated battens, Cobo says, while others think that air flowing through the ventilation channels will be enough to dry the battens out before any decay sets in.
The assembly works in Climate Zone 3, Cobo says, adding that the amount of insulation above and below the sheathing might have to be adjusted for a different climate.
One positive point about the assembly is that it omits a second layer of sheathing, Morgan says.
“Actually,” Cobo replies, “there are metal roofs designed to be installed on substrate and some others on batters or counter-battens; you should check with the manufacturer. You could say not installing a second substrate is a more economical way, or you could choose a better grade and stronger roof that allows installing it on battens. It depends which way you look at it or is available to you.”
There’s no reason to use pressure-treated wood in this application because it’s part of a vented assembly and it won’t rot, says Floris Keverling Buisman.
Notched battens are easy to make
Nor is it necessary to install two layers of furring strips at right angles to each other in order to get soffit-to-ridge venting, says Lucas Durand. On his own roof, in Climate Zone 7, Durand has used notched 1×4 battens, which are easy to make.
“The battens are easily notched with a circular saw while they’re still in a bundle six or eight across,” he writes. “The notch cuts are quick and don’t need to be particularly straight or exactly measured. Lay the battens ‘notch side’ down (they’re for water drainage) and 24 inches on center. Also, the battens don’t need to be nailed down like crazy since the panel screws should be long enough to penetrate through the battens and the sheathing.”
It might be helpful to think of this assembly as a drain rather than a vent, Morgan says.
“For sure, the notches allow air to pass as well as moisture but the purpose is not comparable to the vent channels between insulation and sheathing in a ‘cold’ ventilated roof, which require a much larger opening for the continuous free passage of air,” he says. “Most all sheet metal roofs (other than standing seam) have profiles which permit air movement above the battens: the notches are necessary to permit moisture to drain below the battens.”
A new question pops up
Although this exchange took place several years ago, the question is still of interest to GBA readers.
In a recent post, Douglas Higden raises a new question about the need for ventilation channels beneath a metal roof.
“I’m a little at a loss as to what is special about a metal roof in this regard,” he writes. “I am assuming the metal roof in question is light-gauge steel. Unlike a composition roof which is air, vapor and water tight, steel roofs allow for a lot of air leakage though seams and at the ridge and eave.
“The area at each seam is not in continuous contact with the roof and the pans have minor ribs between the seams which act like rain screen channels,” he says.
The underlayment might be a high-temperature membrane embedded with non-woven fabric that allows the metal to move freely.
“What does furring provide that is not anticipated by the standard, (and usually manufacturer recommended) system?” Higden asks.
Our expert weighs in
Here are some thoughts from GBA Technical Director Peter Yost:
I use the terms “venting,” “ventilating,” and “rainscreen” this way:
- Rainscreen: Synonymous with “drainscreen,” this refers to any roof or wall assembly that passes 90% or more free drainage of liquid water per ASTM E2273. Water-resistive barriers (WRB) like Benjamin Obdyke HydroGap or Tamlyn’s Drainable Wraps qualify. How much air moves through a space of about 1/32-inch, even with the top and bottom of the assembly open? Very little, if any, and that is why these products are not considered either “vented” or “ventilated” (see below). This is the least forgiving of the three rainscreen configurations.
- Vented: Any exterior building assembly with a space between the cladding and the WRB deep enough to allow air movement, but generally open only at the bottom of the assembly and not the top. How deep does the space need to be? Great question. The Canadians settled on 10 mm (3/8 inch) so that free-draining is nearly absolute and air does circulate. In a scale going from least forgiving to most forgiving, this is the “middle-forgiving” of the three rainscreen configurations.
- Ventilated: This is vented with the bottom and top open. You get complete free drainage and airflow because you now have the driving force of the stack effect. This is the most forgiving of the three rainscreen configurations.
Which one do you need? Ah — the answer totally depends on how much moisture you are trying to manage. When I worked for Joe Lstiburek, he asserted that with more than 20 inches of rain a year, claddings should be ventilated. I have never seen research or modeling or field experience to suggest otherwise.
How does this work on roofs? A roof gets wetter than a wall because of where it is, but it also gets more sunlight if it’s pitched.
OK, what about south vs. north exposure and low pitch (< 3:12) vs. higher pitch (>3:12)? South means tremendous drying potential from all that energy and ventilation (free drainage and the drying effects of air flow); a lot less on the north. But with soffit-to-ridge ventilation, I wonder if you can get enough airflow through the ridge on the south side to get a Venturi effect that pulls air on the north side. I need to find some Wingnuts to test this…
So, I think the little spaces created between some types of roof claddings (like a metal roof — standing seam a wee bit and corrugated metal roofing quite a bit more) qualify as a rainscreen roof assembly with good drainage but certainly not 100%.
If there is a dedicated drainage space of at least 10 mm (3/8 inch) with the roof cladding system and open at just the bottom, then I would call that a vented roof.
And if the above applies and the top of the roof assembly is open, then that is a ventilated roof. This is the most forgiving.
Interestingly, the building code says that the roof vent space must be 1 inch deep but I am not exactly sure where that dimension comes from. An old-timer from GAF who has done smokestick field-testing on a whole bunch of roofs tells me that (at least in cold climates) a 1 1/2-inch-deep vent space seems the best. Joe Lstiburek strongly recommends 2-inch depth for serious cold climates.
NOTE: Pressure-equalized rainscreens are a special type of vented cladding configuration, not covered here because they are seldom if ever used on low-rise residential buildings. For more information, see “Pressure-Equalized Rainscreen.”